Which statement correctly defines IDL and MDL?

• A. IDL is the smallest signal distinguishable from noise by the instrument; MDL is the lowest concentration that can be quantified with acceptable accuracy.
• B. IDL is the smallest signal distinguishable from noise by the instrument.
• C. MDL is the largest concentration that can be measured.
• D. IDL and MDL are the same.

Answer: (A)

The key idea is distinguishing two related limits: what the instrument can detect versus what the method can quantify. The instrument detection limit is the smallest signal the instrument can reliably tell apart from background noise—it's about simply deciding that something is present. The method detection limit, on the other hand, is the smallest concentration that can be quantified with acceptable accuracy and precision, meaning you can assign a reliable numerical value to how much is present.

These concepts are different in practice: you might detect a signal above noise (IDL) but not be able to quantify it with acceptable accuracy until you reach a higher concentration (MDL). That’s why the best statement pairs the two definitions correctly: IDL is the smallest signal distinguishable from noise, and MDL is the lowest concentration that can be quantified with acceptable accuracy. The other options either omit one of the definitions, misstate MDL, or say the limits are the same.